The present invention relates to a turbo compound engine of a vehicle, which is capable of recovering the energy of exhaust gas as expansion work of a power turbine and transmitting the recovered energy to a drive shaft such as a crank shaft of the engine by a gear train, so as to rotate the power turbine in a reverse sense upon deceleration of the vehicle in order to obtain braking effort. In particular, it relates to a turbo compound engine provided with a gear train having a clutch to absorb a large load produced when the power turbine is rotated in reverse sense.
Recently in Japan, turbo compound engines which recover exhaust gas energy from the engine as supercharging energy of a turbo charger and exhaust gas energy from the turbocharger as diabatic expansion energy of a power turbine have been developed.
In such turbo compound engines, the power output performance, fuel consumption rate, and gain of the engine are improved by raising expansion ratios of the turbo charger and the power turbine. On the other hand, however, it remains a problem to secure an adequate braking effort (for example, by means of exhaust brake) to counterbalance the increased power output of the engine. In other words, the braking effort against the engine suffers a decrease because of increased turbocharged pressure, so that a main brake (i.e., foot brake) must be manipulated in order to offset the relative decrease of entire braking force. Guaranteeing sufficient braking force is important not only for the maneuverability and safety of the vehicle (engine brake force of approximately more than 60% of the rated output power is required), but also for utilizing the turbo compound engine more effectively.
Thereupon, the present assignee has proposed a "Turbo Compound Engine" disclosed in Japanese Patent Application No. 61-228107 (228107/1986).
In this proposal, as shown in FIG. 3 of the accompanying drawings, a power turbine a for recovering the exhaust gas energy is disposed in an exhaust passage b1 of a vehicle, and a fluid passage c3 is connected to an exhaust passage b2 upstream of the power turbine a so as to bypass the power turbine a. A fluid passage switching means e is provided at the junction of the exhaust passage b1 and the fluid passage c so that it may close the passage b1 upstream of the fluid passage c while opening the fluid passage c when the vehicle is in a deceleration mode and the driving power is transmitted from the crankshaft d to the power turbine a.
This construction makes it possible for the power turbine to recover the exhaust gas energy from the engine so as to utilize the recovered energy as driving energy of the engine during normal driving.
During exhaust braking and when the clutch of the vehicle is engaged, the fluid passage switching means e closes the exhaust passage b1 upstream of the fluid passage c while connecting the passage b2 to the passage c with the junction of the two passages being throttled. At the same time, the rotation of the crankshaft d is transmitted via one of the gear trains to the power turbine a, with the rotation reversed by the gear train. Accordingly, the power turbine a, which is originally designed for energy recovery, performs pumping work i.e., the power turbine compresses the air from the exhaust passage b2 into the fluid passage c. Therefore, it is possible to obtain a large braking effort including motor friction of the engine, negative work upon pumping work by the power turbine, and the exhaust braking force during exhaust braking.
However, the power turbine rotates at a revolution speed ranging from 80,000 to 100,000 r.p.m. during normal driving of the vehicle, and the rotation energy at such a rotating speed is equivalent to the moment of inertia (polar moment of inertia of area) of the flywheel of the normal engine. Hence, when the rotation of the power turbine is switched from normal rotation to reverse rotation, considerable amount of energy has to be consumed somewhere between the crankshaft and the power turbine.
It has been learned through experimentation that the magnitude of the energy becomes maximum when it takes relatively short time (2-3 seconds) from the beginning of the reversing until the power turbine reaches its maximum speed in reverse sense. Thus, what is needed is a turbo compound engine which can absorb the energy by certain means disposed between the crankshaft and the power turbine.
When the rotation of the power turbine is reversed the following shortcomings appear unless the energy is absorbed (in a case where elements between the power turbine and the crankshaft are sufficient in strength).
(1) The vehicle skids momentarily.
(2) An anti-driving force upon skidding exerts an extremely large load on the driving system of the vehicle.
(3) An abnormal abrasion of tires, and brake pads or shoes occurs.
(4) Comfortableness in riding is deteriorated.